1. The locus of the points of application of resultant force in any structure is termed as ___________
a) Pressure line
b) Hollow line
c) Beam line
d) Tendon line
Explanation: The combined effect of prestressing force and the externally applied load will result in a distribution of concrete stresses that can be resolved into a single force and the locus of the points of application of this resultant force in any structure is termed as “pressure or thrust line”.
2. The concept of pressure line is very useful in understanding the concept of ___________
a) Load carrying mechanism
b) Bending mechanism
c) Shear mechanism
d) Torsion mechanism
Explanation: The concept of pressure line is very useful in understanding the load carrying mechanism of a prestressed concrete section, if a prestressed concrete member is cracked, it behaves similar to that of a reinforced concrete section, in contrast to the direct method of analysis of resultant stresses at a section of a prestressed concrete beam outlined the pressure or thrust line concept can also be used to evaluate stresses.
3. The location of the pressure line depends upon __________
a) Breakage and Bondage
b) Magnitude and direction
c) Shear and Torsion
d) Pressure and equilibrium
Explanation: In the case of prestressed concrete members, the location of the pressure line depends upon the magnitude and direction of the moments applied at the cross section and the magnitude and distribution of stresses due to the prestressing force, the increase in resultant forces are due to a more or less constant lever arm between the forces, characterized by the properties of the composite section
4. The eccentricity e in the pressure line diagram is?
a) h/16
b) h/12
c) h/15
d) h/8
Explanation: At the support section, since there are no flexural stresses resulting from the external loads, the pressure line coincides with that of the centroid of steel, located at an eccentricity of h/6, at the centre of the span section the external loading is such that the resultant stress developed in maximum at the top fiber and zero at the bottom fibers and it can be easily be seen that for this section the pressure line has shifted towards the top fiber by an amount equal to h/3 from its initial position.
5. The change in the external moments in the elastic range of prestressed concrete beam results in ___________
a) Bending moment in pressure line
b) Torsion in pressure line
c) Flexure in pressure line
d) Shift of the pressure line
Explanation: “A change in the external moments in the elastic range of a prestress concrete beam results in a shift of the pressure line rather than in an increase in the resultant force in the beam, this is in contrast to a reinforced concrete beam section where an increase in the external moment results in a corresponding increase in the tensile force and the compressive force.
6. The shift of pressure line measured from centroidal axis is obtained as ___________
a) (m/p)-e
b) (m/q)-e
c) (m/r)-e
d) (m/i)-e
Explanation: The shift of pressure line is measured from the centroidal axis is obtained as
E’= (a-e) = (m/p)-e, basically the load carrying mechanism is comprised of a constant force with a changing lever arm as in the case of prestressed concrete sections, and a changing force with a constant lever arm prevailing in reinforced concrete sections
7. The prestressed member undergoes deformation due to the action of ______________
a) Prestressing force and flexural loads
b) Prestressing force and combined loads
c) Prestressing force and transverse loads
d) Prestressing force and tangential loads
Explanation: A prestressed member undergoes deformation due to the action of the prestressing force and transverse loads acting on the member and the curvature of the cable changes which results in a slight variation of stresses in the tendons.
8. The rotation equation obtained by applying Mohr’s theorem considering a concrete beam of span l, force p, eccentricity e is?
a) θp = PeL/2EI
b) θp = PeL/4EI
c) θp = PeL/16EI
d) θp = PeL/20EI
Explanation: A concrete beam of span l is prestressed by a cable carrying an effective force p at an eccentricity e the rotation at the supports due to hogging of the beam is obtained by applying Mohr’s theorem as θp = Area of bending moment/flexural rigidity = PeL/2EI.
9. The cross section of a prestressed concrete beam is 100mm wide and 300mm deep and the initial stress in tendons are located at a eccentricity of 50mm is 1000n/mm2, the sectional area is 100mm2. Find rotation due to prestress (hogging moment)?
a) 0.00155
b) 0.00165
c) 0.00175
d) 0.00185
Explanation: Moment of area I = (100×3003/12) = 225×106mm4
Prestressing force p = (1000×100) = 105 = 100kn
Rotation due to prestressing force θp = PeL/2EI = (100×50×6×103/2×36×225×106)
Hogging moment = 0.00185radians.
10. In the elastic range, any increase in prestressed member does not show any effect on ____________
a) Steel stress
b) Compressive stress
c) Bending stress
d) Flexural stress
Explanation: In the elastic range any increase in loading on a prestressed member does not result in any significant change in the steel stress and in other words, the stress in steel is more or less constant in elastic range of prestressed concrete